A number of intrusion detection systems have been proposed using a sensing unit having two motion detecting sensors and processing the signals from these motion detection sensors to produce an alarm signal when appropriate. Typically, signals produced by the sensors within a predetermined time period of each other, indicate a confirmed event and result in an alarm signal. Some sensing units produce a trouble alarm based on certain characteristics of the responses received from the motion detection sensors other than a confirmed event and often are identified as unconfirmed events. Examples of such prior art systems are U.S. Pat. No. 4,710,750 (Johnson), U.S. Pat. No. 4,195,286 (Galvin), U.S. Pat. No. 4,611,197 (Sansky), and U.S. Pat. No. 4,833,450 (Buccola et al).
Such systems produce an alarm signal based on a confirmed event or produce a trouble signal based on some processing of the signals received from the motion sensors based on unconfirmed events. Unfortunately, these systems do not allow the user to significantly vary the characteristics of the sensing unit to suit his own needs or to suit the particular environment in which the unit is being placed. For example, in monitoring of certain space, a very high degree of security may be required where it would be worthwhile if the sensing unit could produce an alarm based on confirmed events or produce an alarm based on certain characteristics of the unconfirmed responses received from the individual sensing units indicating that the unit may not be working satisfactorily or that environmental conditions are creating spurious indications of motion for either one of the sensors. In other environments it may prove particularly bothersome to produce an alarm based on unconfirmed events and it would be much more desirable merely to produce a trouble signal which can then be investigated by the user. Furthermore, it would be desirable to be able to have a system where the user has much more control with respect to resetting of the sensing unit.